1. Field of the Invention
The present invention relates to an optical waveguide device used for an optical communication, and in particular, to a Mach-Zehnder optical modulator.
2. Description of the Related Art
An optical waveguide device using electro-optic crystal, for example, lithium niobate (LiNbO3), lithium tantalate (LiTaO2) or the like, is formed such that a metallic film is formed on a part of a crystal substrate to be thermally diffused, or proton exchanged in benzoic acid after patterning, to form an optical waveguide, and then an electrode is disposed in the vicinity of the optical waveguide. As one of such optical waveguide devices using electro-optic crystal, there has been known a Mach-Zehnder optical modulator with branching interference type optical waveguide structure.
FIG. 8 is a perspective view showing an example of a conventional Mach-Zehnder optical modulator configured using a lithium niobate substrate of Z-cut. In this conventional Mach-Zehnder optical modulator, a titanium (Ti) film is formed on a substrate 101. The substrate 101 formed with the titanium film is patterned into a shape of Mach-Zehnder type, and thereafter, heated for 7 to 10 hours at 1050° C., and thermally diffused. As a result, an optical waveguide 110 is formed. The optical waveguide 110 comprises an incident waveguide 111, a branching section 112, parallel waveguides 113A and 113B, a multiplexing section 114 and an emission waveguide 115, and a coplanar electrode 120 comprising a signal electrode 121 and an earthed electrode 122 is disposed along the parallel waveguides 113A and 113B. In the case where the substrate 101 of Z-cut is used, the signal electrode 121 is arranged over the optical waveguide 113A in order to utilize a change in refractive index due to an electric field in a Z-direction. Further, the signal electrode 121 and the earthed electrode 122 are formed on the substrate 101 via a buffer layer (not shown in the figure) consisting of SiO2 having the thickness of 0.2 to 1 μm, so as to prevent the absorption of lights propagated through the parallel waveguides 113A and 113B.
In the case where such a conventional Mach-Zehnder optical modulator is driven at a high speed, one end of the signal electrode 121 is earthed via a resistor (not shown in the figure) to be made a traveling-wave electrode, and a high frequency electric signal S, such as a microwave, is applied with through the other end of the signal electrode 121. At this time, since the refractive index of each of the parallel waveguides 113A and 113B is changed due to an electric field E generated between the signal electrode 121 and the earthed electrode 122, a phase difference between lights being propagated through the parallel waveguides 113A 113B is changed, so that a signal light L′ whose intensity is modulated, is output from the emission waveguide 115.
For the Mach-Zehnder optical modulator as described above, It has been known that a cross sectional shape of the signal electrode 121 is changed to control an effective refractive index of the microwave, and propagation speeds of the light and the microwave are matched with each other, to thereby obtain a wide band optical response characteristic. Further, there has been proposed a technique in which an earthed electrode is disposed on a rear face (opposite to the surface on which the optical waveguide 110 and the electrode 120 are formed) of the substrate 101, on a side face, along the parallel waveguides 113A and 113B, of the substrate 101, or the like, to achieve the stabilization of a propagation characteristic of the electric signal S to be applied to the signal electrode 121 (refer to Japanese Unexamined Patent Publication No. 10-239648, Japanese Unexamined Patent Publication No. 2003-75790, Japanese National Publication No. 5-509415 and Japanese Unexamined Patent Publication No. 7-64030).
In the conventional Mach-Zehnder optical modulator as shown in FIG. 8, there is a problem in that the optical response characteristic is deteriorated in the case where the electric signal S at a high speed of for example 40 Gb/s or the like is applied to the signal electrode 121. Namely, if the electric signal at 40 Gb/s or the like is applied to the signal electrode 121, a dip occurs in a frequency characteristic (S21) of the electric signal S being propagated through the signal electrode 21 as shown in FIG. 9, caused by the resonance of a certain frequency component within the substrate 101. Due to the occurrence of such a dip, a loss of an optical signal being propagated through the optical waveguide 110 relative to a data signal having a particular pattern corresponding to that frequency becomes large, resulting in the deterioration of the optical response characteristic.
Further, in the case where the propagation characteristic of the high frequency electric signal S is improved by disposing the earthed electrode on the rear face or the side face of the substrate 101, there is a problem in manufacturing as follows. Namely, in the case where the earthed electrode is disposed on the rear face of the substrate 101, it is difficult to perform a visual inspection from a rear face side. To be specific, in many cases, the displacement of the optical waveguide 110 and the coplanar electrode 120 formed on the surface of the substrate 101 is usually verified by the visual inspection from the rear face side of the substrate 101. However, if the earthed electrode is formed on the entirety or a part of the rear face of the substrate 101, a state of the surface side is invisible and therefore, it becomes difficult to perform the visual inspection as described above.
Moreover, in order to reliably earth the electrode formed on the rear face or the side face of the substrate 101, it is necessary to form, for example, a metallic film continuously from the earthed electrode 122 on the surface through the side face to the rear face. However, since the formed metallic film is likely to separate from the face in the vicinity of corners of the substrate 101, there is a disadvantage in that the yield rate and the reliability of the substrate 101 shall be reduced. Although there is a configuration in which the electrode on the rear face or the side face of the substrate 101 soldered on an inner face of a housing to which the substrate 101 is implemented, to be earthed, generally, a gap between the substrate 101 to be implemented and the housing is very narrow and it is not readily to perform the soldering or the like. Therefore, there is a problem in that the reproducibility of the earthed state of the substrate 101 after implemented is low.